The present invention pertains to the welding of pressure vessels and, more particularly, to apparatuses and methods for friction stir welding of pressure vessels from the inside.
Friction welding is based on the principal of xe2x80x9crubbingxe2x80x9d together two articles to be joined so as to generate a sufficient amount of heat and plasticize the adjacent surfaces. The frictional heat is generated solely by the adjacent work pieces to be joined.
A more robust version of friction xe2x80x9cstirxe2x80x9d welding utilizing a rotating, non-consumable probe or pin has been developed. The pin is of a harder material than the work pieces to be joined, and relative cyclic movement of the pin generates the frictional forces to plasticize the workpiece material. Because of this movement of the non-consumable pin, the method has been named friction stir welding, with the pin xe2x80x9cstirringxe2x80x9d the material. Aluminum is a particularly suitable soft material that can be readily friction stir welded. The aerospace industry is aggressively pursuing friction stir welding of aluminum panels for the outer skins of launch vehicles because of the weld quality and repeatability.
In a conventional butt joint, for example, the pin spins or linearly reciprocates and travels along the interface between the adjacent or xe2x80x9cfayingxe2x80x9d work piece surfaces to create the plasticize weld joint. Typically, the two work pieces are placed on a backing surface and are tightly held together to prevent separation during the friction stir welding process. Further, a stationary shoulder surrounding the moving pin applies forging pressure to the area around the faying surfaces on the opposite side from the backing surface to prevent the plasticized material from extruding out of the joint. The result is a relatively smooth and void-free joint.
Friction stir welding, because of its many advantages, is being considered as an attractive process for the assembly of aluminum tanks, particularly large aluminum alloy pressurized tanks, such as those utilized for cryogenic oxygen storage in space launch systems. Because of the large compressive forces involved in friction stir welding, the final welds in these tank structures present a problem of how to adequately provide backing support inside the tank, particularly when access to the tank interior is restricted. Outside diameter (OD) circumferential welding requires tooling to be erected inside the pressure vessel so as to provide a path for the friction stir welding spindle axial load from one exterior side of the vessel to the other where a rigid fixture is provided. The installation and removal of tooling from inside of the tank is time-consuming and may damage the tank.
One technique for friction stir welding of tanks is disclosed in U.S. Pat. No. 5,697,511 to Bampton, wherein an internal disk-shaped support is provided within the tank along the weld line. The internal support provides backing for the external friction stir welding tool, and coalesces into the weld line to become an integral part of the tank pressure vessel. This undesirably adds to the weight of the finished tank.
There is thus a need for an improved apparatus and method for welding large pressure vessels.
The present invention provides an apparatus for function stir welding a seam in a container from the inside, the container being of the type having an opening to the inside. The apparatus comprises a convertible welding head having at least one friction stir welding (FSW) spindle mounted thereon adapted to passed through the opening in the container with the FSW spindle in a retracted state and weld the seam from the inside of the container with the FSW spindle in an expanded state. The weld head is too large to fit through the opening of the container with the FSW spindle in its expanded state. Preferably, the weld head includes a plurality of FSW spindles.
In one embodiment, the weld head has a central frame and a force balance arm convertible between a retracted state and an expanded state. The FSW spindle extends outward from the frame in a first direction in its expanded state, and the force balance arm extends outward from the frame in a second direction different than the first direction in its expanded state. The weld head may also include a second FSW spindle and a second force balance arm, wherein in its expanded state the second FSW spindle extends outward from the frame in a direction opposite the first direction, and in its expanded state the second force balance arm extends outward from the frame in a direction opposite the second direction. Preferably, the two FSW spindles and two force balance arms extends outward from the frame in substantially the same plane, and the oppositely-directed spindles extend along a line that is 90 degrees offset from a line along which the oppositely-directed force balance arms extend.
Recesses in the frame for receiving the FSW spindles and force balance arms in their retracted states may be provided to reduce the profile of the weld head. An apparatus for converting the FSW spindles from their retracted to their expanded states, and vice versa, may include a piston/cylinder mechanism. Furthermore, one or more sensors or cameras may be provided on the weld head to facilitate positioning of the FSW spindle with respect to the container, and/or to monitor the friction stir welding operation.
In accordance with one aspect of the invention, the FSW spindles pivot with respect to the central frame from their retracted states generally adjacent to the central frame and aligned along the axis, to their expanded states pivoted about 90xc2x0 away from the central frame with respect to the axis. The central frame may house at least one motor for rotating the FSW spindles.
The present invention may also provide a pair of headstocks linearly movable along a common axis. Each of the headstocks has a weld head mounted thereon, the weld heads extending toward each other along the common axis. With this arrangement, two seams in the container may be simultaneously welded using the two weld heads positioned inside the container. In a particularly efficient application, the container is a least partly tubular and the seams are circular about the ends of the container. A stand for supporting and rotating the tubular container is provided, wherein the weld heads remain stationary in their expanded states while the tubular container rotates therearound to enable welding of the circular seams.
In accordance with a further aspect of the invention, an external anvil support is provided around the container at the seam to supply a backing surface for the internal friction stir welding operation. If the seam is circular, the anvil is a circular hoop.
The present invention also provides a method for friction stir welding a seam between two sections of a container, the container having an interior space and an opening thereto defining an axis and being smaller than a cross-section of the interior space normal to the axis. The method includes providing a convertible weld head having at least one FSW spindle mounted thereon adapted to pass through the opening in the container with the spindle in a retracted state. The weld head is too large to fit through the opening in the container with the FSW spindle in an expanded state. The method further includes passing the weld head with the FSW spindle in its retracted state from the outside of the container to the interior space through the opening. The FSW spindle is displaced from its retracted state to its expanded state, and the seam is welded from the inside of the container.
The container may include a tubular section, wherein the seam being welded is circular. The method therefore may include causing relative rotation between the tubular section and the weld head during the step of welding the seam. This relative rotation is desirably caused by rotating the tubular section about the axis of the opening while holding the weld head stationary. Desirably, the method further includes externally supporting the container at the seam during the step of welding the seam, and more preferably providing an anvil hoop around container at circular seams.
The method may further include providing a second convertible weld head similar to the first, passing the two weld heads with their FSW spindles in their retracted states from the outside of the container to the interior space through respective openings, displacing the FSW spindles in the weld head from their retracted states to their expanded states, and simultaneously welding the two seams from the inside of the container by rotating the tube about the axis while holding the weld heads stationary.
A further understanding of the nature advantages of the invention will become apparent by reference to the remaining portions of the specification and drawings.